This invention relates generally to thermostats, and more particularly to controlling a fan with a thermostat.
Known thermostats employ a resistive element to reduce or prevent overshooting a temperature setting. The resistive element is typically referred to as an anticipator. Thermostats without anticipators sense a temperature one or two degrees above a temperature setting and then open a switch contact. For instance, in an unanticipated thermostat set for turning on a furnace, the switch contact will remain closed so the furnace continues to run until the temperature rises one or two degrees above a temperature set point. The temperature rise above the temperature setting is caused by the delay in heating the thermal mass of a bimetal heat sensing element, located in the thermostat, above the set point temperature, and this excess temperature rise is known as overshoot. When the temperature rises above the temperature set point, the switch contacts open turning the furnace off. Then as the room temperature decreases, the temperature has to drop one or two degrees below the temperature set point before the switch contacts are closed to turn on the furnace. This temperature drop below the temperature set point is known as undershoot. Typically the anticipator functions to minimize undershooting and overshooting the thermostat""s temperature set point.
The anticipator xe2x80x9canticipatesxe2x80x9d when the room temperature approaches the temperature set point of the thermostat. An anticipator is a resistive heating element. When a thermostat is turned xe2x80x9conxe2x80x9d for heating, a current is applied to the anticipator. The current flow through the anticipator heats the anticipator which is electrically connected to a bimetal switch. The bimetal switch deflects with temperature changes to open or close the contacts. When the room temperature decreases below the thermostat set point, the bimetal switch contracts and closes the contacts turning on the furnace. The anticipator xe2x80x9cheatsxe2x80x9d the bimetal switch to sense a higher temperature within the thermostat compared to the room temperature. By adding internal heat, the anticipator reduces the amount of room temperature differential required to turn off the furnace.
However, in such a system employing an anticipator, the connection of the anticipator across the contacts of the thermostat does not allow the voltage across controlled elements, such as a relay coil, to go to zero during the off state. Instead, a finite voltage remains across the relay coil when the thermostat is in the off state. Lower resistance of the relay coil compared to the anticipator, results in a lower voltage across the relay coil. In known systems, the relay coil applies electrical power to a heating or cooling system. However when the thermostat is in the off state, there exists a finite voltage across the relay coil less than an amount of voltage required to energize and close the relay coil. It would be desirable to coordinate the finite voltage across the relay coil to that voltage required for the activation of a variable speed fan. If would be further desirable for an electronic circuit to provide an interface to electrically couple the different operating voltage requirements between a relay coil and the variable speed fan.
In an exemplary embodiment, an electric circuit is connected in series to a thermostat including a bimetal actuated contact with an anticipator across the contact. The electrical circuit including a pair of optically coupled isolators to allow a variety of input control options. The options include the selection of different air flow rates. Either of the pair of optically coupled isolators maybe connected to the thermostat. Power to the electric circuit is provided by voltage developed across the system relay coil in series with the thermostat, e.g., current flows from a system control transformer though the thermostat contacts and through the relay coil. The voltage generated across the relay coil is used to power the electric circuit to electronically control a fan.
The electrical circuit includes in one embodiment, a first input terminal and a second input terminal, which are connected to one optically coupled isolator. Connected in series to both the first and the second input terminals is a first zener diode that establishes a threshold voltage consistent with the operation of the system relay coil and the thermostat. In addition, a second zener diode connected in series to the first input terminal rectifies the AC voltage to a half-wave rectified voltage. Connected to the second input terminal is a fall-wave bridge rectifier that rectifies an AC voltage to a DC voltage. A purpose of the first zener diode in series with the first terminal is to generate a half-wave rectified voltage for the optically coupled isolator to differentiate the input voltage from the first input terminal from the input voltage from the second input terminal. In addition, the first zener diode in series with the first terminal protects the electrical circuit from transient voltages, e.g., electro-static discharge voltage. In addition, the optically coupled isolators provide the electrical isolation to protect the thermostat, including a ground reference control circuit, from high voltage circuits that provide power to the electronically driven fan.
More particularly, the fan control is electrically connected to an electrically erasable programmable read-only memory (EEPROM), which is programmed to control a plurality of fan modes based on combinations of inputs from the thermostat. As a result, a cost-effective and reliable electrical circuit including optically coupled isolators to control fan activation is provided .